One simple thing to keep in mind: The coarser the sandpaper, the straighter the cut. Don't be afraid to start blocking your body filler with 36 grit to rough it in quickly.
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View attachment 1715186678 View attachment 1715186679 I like to block my high build primer with 180 grit dry to get it close, then finish it off with 320 dry. Use a dark guide coat! You can make your own. I usually just take the last little bit of high build primer that's left in my gun and add a little bit of black color (basecoat, enamel or even spray can stuff) right into the mixed up primer. Then I add more reducer to make it nice and thin, like water almost. Spray a light mist of that over your fresh primer and then your guide coat is ready to go.
Good luck!